This project will develop a nonmetallic temperature probe using a small optical etalon as the sensing element. The etalon -- an optical flat coated to form a reflecting cavity -- is connected to the light source and detector by a fiberoptic bundle. Important biological applications of such a system include monitoring of microwave biohazard studies, microwave thawing of frozen organs and blood, and possible microwave therapy for cancer. In all these radiation experiments, conventional thermistors, thermocouples, and their metallic leads cannot be used because they significantly perturb the electromagnetic fields and cause attendant temperature errors. The optical technique relies upon the change in the wavelength-dependent reflection from the face of the etalon as the temperature varies. Temperature changes cause thermal expansion of contraction in the etalon material as well as changes in the index of refraction, thus varying the resonant wavelengths. A similar technique, based upon the temperature shifts of the band energy gap in semiconductors, which can be sensed optically by measuring the wavelength of the adsorption edge, will be pursued. Prototype probes are under development by evaluation testing.